1. Field of the Invention
The present invention relates to a liquid crystal display device having a poly-silicon TFT substrate in which a condenser type digital analog converter is formed.
2. Description of Related Art
A liquid crystal display device having a poly-silicon thin film transistor (hereinafter referred to as TFT) substrate in which a plurality of poly-silicon TFTs are formed has been widely used as a display screen for use in personal computers and the like, which is a large-sized device, as well as a display screen for use in portable devices and the like, which are a small-sized device.
Recent years, a price reduction of the liquid display device has been demanded according to developments in price competition in products incorporating the liquid crystal device. To cope with this need, the development of liquid crystal display devices, which incorporate a condenser type digital analog converter (hereinafter referred to as a C-DAC) for converting image data of a digital signal to an analog signal, has been advanced.
The condenser in the C-DAC requires a certain time until a voltage is stabilized after turning-on a switch for the condenser. Assuming that this time be τ, a time of n·τ is necessary for a C-DAC of n (n: a positive integer) bits.
Incidentally, the liquid crystal display device has a constitution that holds liquid crystal between a poly-silicon TFT substrate and an another substrate disposed so as to face the poly-silicon TFT substrate, and can be considered as being a kind of condenser composed of the poly-silicon TFT substrate and the another substrate. Assuming that the electrostatic capacitance between the poly-silicon TFT substrate and the another substrate be CLC, a resistance of a light blocking film disposed on a surface contacting liquid crystal of the another substrate be RBM, and the time constant of the electrostatic capacitance be τLC, the relation expressed by τLC≈RBM×CLC is established. It is sometimes inevitable owing to limitations in circuit design that the time constant τLC is equal to n·τ or less.
However, when the time constant τLC is equal to n·τ or less, potential of the condenser of the C-DAC varies during an operation of the C-DAC. Therefore, there has been a problem that a stable output corresponding to image data of n-bits cannot be obtained from the C-DAC and display quality is degraded.
Particularly, in the case that the size of the liquid crystal display device is small, the relation expressed by the equality: CLC=k·S·L is established assuming that a diagonal size of the liquid crystal display device be L, the number of scanning lines be S and a coefficient be k. Then the electrostatic capacitance CLC becomes smaller, as the size of the liquid crystal display device is made smaller. Since the time constant τLC becomes smaller as the electrostatic capacitance CLC becomes smaller, the time constant τLC is apt to be equal to n·τ or less, and there has been a problem that the C-DAC does not correspond to the number of bits of the image data and image quality is significantly degraded.